There are many instances wherein it is necessary to accurately deal with small voltages which are encumbered by high frequency noise and the presence of high, and conceivably, variable, other common mode voltages. An illustration of this would be in the monitoring of current through a high voltage conductor of a high voltage direct current power line, such as that emanating from a large scale photoelectric solar array. Typically, in such case, current would be sensed by a small value resistor in the power line, with the voltage across this resistor being indicative of current. Assume, for example, that the actual supply voltage on a power line is on the order of 500 volts and that small voltages, representative of current through the power line, are sampled and that it is desired to amplify these small voltages and provide an output signal representative of them which is referenced to a ground or other reference with respect to which the 500 volts on the line appears. This obviously requires measurement circuitry which must be responsive to small voltages but be able to withstand the presence of a higher one. This suggests the employment of some form of isolation between elements of the measurement system and the high voltage environment. In accordance with this approach, one technique has been to employ Hall effect devices. A second approach is to isolate the sense signal from the analog output signal with a converter using a transformer. A third approach is to use an analog-to-digital converter with optical isolators between the sensed signal and the output. The Hall effect device and the transformer passing on analog signal and the optical isolators passing digital signals will withstand the high and low common mode voltages acceptably. None of these approaches have proven optimum in terms of cost, accuracy, part count, and effective range of operation.
It is the object of this invention to provide an alternate approach and one which employs only simple and inexpensive electronic components, namely, only resistors, standard operational amplifiers, and low cost transistors. In addition to the lower cost that necessarily results, it is the further object of this invention to enable extremely accurate measurements over quite acceptable ranges of measurement.